BSTJ 60: 7. September 1981: Digital Signal Processor: Voice-Frequency Transmission Treatment for Special-Service Telephone Circuits. (Blake, R.B.; Bolling, A.C.; Farah, R.L.)

14.4 MB·English

Digitat Signal Processor:

Voice-Frequency Transmission Treatment for
Special-Service Telephone Circuit

By A. 8. BLAKE, A.C. BOLLING, and RL. FARA
(Manuscript recetved Jury 30, 1980)

This article describes the application ofthe digital signal processar
ea voiceband signal processing element, The application choven it
one of the most stringent noire frequency signal procesting appa

tions in the telephone network providing transmissian treatment
(em, gain, equalisation, and echo control jor special aervice circuits

A vetted description of a protatype transmission tratisent uni,
‘which uses dhe Dap. ie provided along neh descriptions of the digital
Fitter structures and filter ayntheots techniques. Measured results for
repreventatice extreme cable facility cases are presented, showing
that digital signal processing utilizing the use meets the telephone
network transmission requirements fur ypecial sevice cirewts,

1. wTRODUCTION
‘The frst, and still most heavily «sed, (ransmission medium in the
felecammunications network is copper wire, Twisted pair metalic
cables of various gauges, lengths, and sees make up the bulk of the
loop plant and local exchange trunk plant. The pplietion of electronic
amplifies or repeaters to provide gain to compensate for Ihe wen
{ation of signals on mcallio eables ws the ft large-scale use
sleccronies in the celecommunicaLions network, Repeaters ate sil
sed extensively. By necessity, che systems of the pase chat proved
voice-requency (vr) transmission irealmenl on 8 per-channel Basis
‘were analog aystems, The lange-acale use uf sophistented dig signal
Drocedsing for these sransmiseion treatment functions was prelate
By cos, power consumption, al ste of amplementation. However, in
recent years the capabilites uf digital hardwaze have iproved
nificantly, primily ms result of the rapid development of integrated
‘Sreut technology.

"ha digital signal procesor (nse, a vist device with large umber
oflogie erie, ivan example of dhe sophistication that is now posse
‘vith digital hardware, The inherent capabilities of the ost have made
it posible to consider the se of digital flters to replace some of the
‘raditionl analog network functions in vr transmission systems. This

per reports on the results af an experimental try in which he one
fn sed for this purpose. Amung the ost desirable features of the sr,
and sqpecaly important for this study, isthe ease with which ite
be programmed under compuler ennrol to provide transmission tre
ment functions,

"The principal tansmision signal pracesing fonctions are performed
in ve repenters for metallic tenamission systems and in carrier ler
‘minal units (Crus) fr tranemieson ayers Chat contain a metali-to-
‘arror interface. Taste syseraa provide signal processing fora variety
of services Typically, the most demanding sxnal processing require-
‘ments derive from special services applientinn" Special aervice eteuits
fae enyincered using all pes of transmission medie. Special services
fre large and rapidly growing pare of the ‘clecommunieations net-
ror

Performance objectives for a special service creuit are normally
spetified in terms of kHz oes attenuation distorion, eco dacorion,

nd various oer teanemiesion parmmeters. 1 signal proceasing faust
The ficluded in a eieuie £0 that {ranisson objectives for ehe creat
fan be achieved, che eicuit is called » treated cireuit. ‘The prototype
‘ananiasion teatment unit, doscrhed in thi article, whieh uses the
Dro provide adjustable tranenision treatment fanetion, is referred
tos digital resent transmission amit (0°70). The description of
the v7 and ite capabilites gine in Section LL Fir, in Section I
“nn oulline of treated circuits is prevented sinc the performance goals
for the DTT were based on the performance abjoctives for these
circuits

1. INTRODUCTION To VOICE-FREQUENCY CIRCUITS

“Table Llsts some ofthe applications for metallic ceatment systems
Figure 1 llustrates some of the possible circuit arrangements with
tangas treatment, Each of the arrangements is assumed to be
providing a special servi cicuit—in this ene « foreign exchange
trunk. Figure la shows termingl repencr, with a switching system.
timane side and a2-vir ele connected to a PRK on the other. Figure
This amila, but with the repeater placed at an intermediate location
inthe circuit Figure Te sows the use of to repeaters na long cuit.

Finally, Pig, 1d shows 2 cteut that contain both aeazter lin and

1886 THE BELL GYSTEM TECHNICAL JOURNAL, SEPTEMBER 1851,
Table ImInserted Connection Lows 1)
objectives for typical 2-ire itched
special services:

eneee|

ee

1 Devo cab facility. This latter configuration, with a earir link in
terudem with a 2-wire metallic estension, ix quite commen, Cartier
systems are designed to be ersentilly (rparent for transmission
ppurpmes Noto cht ifthe carrer Ink is removed in Pig. 2d end the
rvs retained, the rolling circuit amangenent is xinilar to that
shown in Fig. 1a,

"The crv that provide the 4- to 2wite interfaces must provide

VOICE-TREQUENCY TRANSMIGSION 1587
eanamission ereutment. fir ehe adjacent metallic cable. Carrer termi
dl unite with analog reatment ze currently available for both iit
nd. analog carrier syst, A digital carrier ayetem is un obvious
Candide for dil traneriaion treatment since digital unending and
tleooding of signals ie alrendy required. Digital switching. systems,
‘whieh have transtission characteristic similar to dial carsees are
lo strong camfidaten for digital treatment of special service circuits
"This article concentrates on the digital to-analog 4 2wine incertae.

2.1. Tranamiesion veatmint objectives

"The auecess of the irlecontununiestions nevwurk in providing atis-
factuty service places requirementa or objectives on some ofthe base
tlt ircal charaereistios nf the equipment wit. The service sted in
‘Table Tort be wngineeted to have earehlly vontolled ansmission
properties ‘The mnwin theme of this study i the use of the nse to
Eontol the loss attenuation dislrlin, and echo distortion of imate
Clreuile. This iv accomplished hy raplerarntng digital fers with the
Da. as is described in detail in tho following sections. AlUhongh not
Considered here, the osv could wbo be programmed to contr elay
Aistoriion of x treated acu.

‘Ofte service Hsted in Table , che trunk have dhe most stringent
performance objectives A typical shore bal treed special service
Trunk must be engineed to have a LAH los of 35 dB to en noeuracy
bt approsimately 0.8 #B, Furtheraore, The lose of the eireit at <i)
tod 200 Hi relative tothe 1-KHz les, should he within the fellawing
limits: a 400 Ha, ~L0 dR wo 80 dl; at 2800 He, ~1.0 dB to 4. dB,
Aulitionlly, the eireult Joa should be relatively smooth between
‘hone ovo froquoneis. The cieuit should ssn he designed co chat its
lose response rolls iff tthe vv band edges o enbancestaility margin
sn echo performance. For hw eniguration shown in Fig. 1a, emstalh
Considratinn retret the gin of the repeater to a maxim af 6B,
1 kM, and the loge ofthe able to a maximum of dB at 1 ki

Since u 2-wire treatment unit is inherently a feedback device, some
‘means rust be provided to “balance” the unit, tha i, significantly
Fede che feedback, io ensure adequate stalihiy margin and satistac
tory echo performance for a treater cirmit. Tn the DITU, a digital
Faneeler network provides this balance furetion. Since gain adel 10
flreut amplifisan echo along with the denzedsigasl, erento circuits
fost be beter balanced shan unleeated cizcuits. Poor lar ests
in listener distortion on short civics end caller echo on lng eieuita.
‘Ava 4 to -wie interface, Uhe equipment should produce atleast 15
fob a8 of los between the 4-vire porta (eg, porte sand 2 Fig. 21,
Incuding Un: eects of gun, over most of che voiceband. The Ins at
the vv band edges may be somewhat les, although adequate stables
‘margin mast be rineaine.

11680 THE RELL GYSTEM TECHNICAL JOURNAL, SEPTEVBER 198
[Oo "eine

Poly oir a ache fle ie 159 42 as beren

1M, DIGITAL TREATMENT TRANSMISSION UNIT

A\ bloc diagram of the D770 is shown in Fig. 3. "The digital ports of
the ovre eval interface witha digital erser oF a digital switch, while
the 2owire analog por: could interface with logp plant cable or trunk
‘able. The DTTC consists of @ DSP, ine interface unit (170), nel the
“approprinte lgic devices to interface the os tothe 10. All xjuntable
{in, equalization, and echo cancellation are provided by digital filers
lmplemented with the nse. The remaining DTT functions, auch an
‘encoding and decoding of signals are proved in the Li.

3.1 Line intertsce unt ranamision functions

The experimental version ofthe LIU contain a commercially aval
able 16 fal Knenrdigital-n-analogcouvertar, Unpublished wuicn

&

4

YOICE-FREGUENCY TRANSMISSION 1860
by J.J AW. Unger have sown thet she standard 1-255 encoding cnes
‘not pride sufficlene dynamic range for digital signal proccss
ome special service appliations. The 16-bit hinear enced
‘hove to avoid this problem, ‘The convertor ie followed by a recon
“ruction filter that removes energy above 4 KEfz. The reennamuction
filter driven a fixed-pain anslog amplifier that, in turn, dives the 2-
cable throngh a M-ohm teansformer-coupled output. Addition.
the amplifier drives an analog compromise cancaler, a device
fiscunsed in more detail inthe next paragraph. Input signals from the
fable are coupled through the transformer lo a diferential ampliiee,
‘The eitferential amplifier drives an antialiasing ver that. removes
feneris above 4 Kilz and sn removes 6-1 Induction. The oucput of
the antialiasing iver is sampled at 8 KHz snd then concerted co digital
form by ¢ 16-bit falinese analog-to dlgial converter.

"The primary purpose ofthe compromise canceler isto provide some
lose tappreximacely 6 aH or gresver) between points “a” and "Bi of
the LT for the universe of cublen with which the prTU is expectei
Interface, Is transfer function is Bxed, with one polo end no wos. AS
compared 10 the ne performance in the abeence af the compromise
tanceler, the henefits obtained are (wnfold

Te large signal se point "ais let Hikely co overload the analog
tondigial nvertor

Tis The performance of the diyilal echo canceler in the DAP is
enhanced.

2.2 Diptaeanalproceesartrenariasion functions
As mentioned above, all adjustable gain, equalization, and echo
cancellation for the D790 are provided by digital flere in the nse.
‘These fiters xe:
1 an equalize forthe transmit rection of transmissnn,
(i) an equalizer forthe recive divection of transmission, and

ia) a canceler,

‘The equalizers provide adjustable gain as woll ws adjustable equal
zation. "The trefor functions of thebe three Miers are dentoed by E,
5, and C, respectively: There filvers wil be represented in many of
the remaining figure in Cie artice bythe symbols shown in Hig 4

|W. SELECTION OF FILTER FORMS FOR THE EQUALIZERS AND

"The neve must have fhe capeblity to provide tronsmicion tout
sent for large varity of cable facilities, Laboratory and computer
‘Stuulationsf a representative aample of treated trannisionfacities
Ihave ahown that the OTe hae the required capability if the eanceler
su S2ap transversal Fler and each: equalizer is a fourth-order

1590 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBE 1201
Fig. «Syme rprsntain af re dtl er

recursive filer composed of two cacaded biquads. In Z-transforms
rotation, the Ger finetions are
0-8 me",

omy Sachse tate yy
ey Sets tae &

TH b= Baie
‘Symbolic diagrams ofthese filters are shoven in Figs $ an 6. (ry the
Figures, 5,7, and vs reprosont signal values, and are ced for the
iscustion in Seetion V.) The equilizer premuluphers, Gy and Gr
allow flesbility in parceling out yin belorden the ter sections and in

ania

VOICE-FRrQUENGY TRANEMISsICN 1504
controling corficiene magoituds, which is constrainei ly the se
oeticient slorage format to be le than 20, ‘The coficienss in the
thrve equations above ae different foreach differen eal fic.

‘A recursive sruciur, rather than transversal stritine, suse for
the equalizers for one primary Yeasor: in addition vo providing gain
fan etalization it the frequency bxevd 400 and 2900 Lx, for 2-wire
‘ranemiasion it volo necessary thatthe equalizers provide significant
sttenuation athe ¥F band edges to enhance che stability margin
pesformance ef the Dru, This charwolerstic i moze resdily obtained
{witha low-order rsursive tract, fur he eensmission system under
‘onsderasion, than it is with a reasonable ize transvrratl er

"A cneaded biquadstuotare rather than a dliret-form fourth-order
saracue, is uso! for the equalizers became the transfer fanction
the easeeded strucre is Int sensitive lo coelfiient quantization. In
Feta, 4 and 6 i is show tha if the unquantized poly locations oF
zee locations lle o each other, ceMicen- quantization can cause
‘tovge shift in the pole or taro leations, I Ue sitt occur, the filter
fesponve wil differ from the desi response. In the direct-form
Fealizstion. the shit as 4 funtion of eveficient quuntization is de
pendent of sll the pole of zero locations. Howeee, in the cascade
Form. the pole or zero aif i one section is indepen. ofthe pole or
ev locas in the other sein

\, FILTER INPLEMENTATION WITH THE OS?

In this vetion, the implementation of the equalizers and canceler
with che vot is disoused, Fora description of the nsP wehiteccure see
[Ret 1. Thorn are four tops of nieve with regard to the nse program.
‘These ure

41592 THE BELL SYSTEM TECHWAL JOURNAL, SEPTEMBER 1981
1 Location of eoetvins andl daly ithe a0,
(i) fer implementation,

Ain) coetficen loaing, and

lie) program storage ad exoewtion tin,

5.1 Digit! strat processor RAM mamory map

Sinee the filter ooeficionts are different for each diferent. cable
facility, the must be stored in the Bs Kaa, Ala, che delayed signal,
luca must be sored i RAK. Coecients and delayed signal values
fro scored a ahown in Fig. 790 a Wo minimive the number of reine
ets vequized fo access them,

‘5.2 Filter implementation

5.2.1 Equalizers

Tn the ust program used for the nr70, the three filtering operations
are performed inthe folowing onder

re
Wa 6,

Gin B
‘Thecanceler is dices below, in this section F, and Bare discos,

ince the program stop used to implement and Bate identical, «
seneraldesenption of the Gltering steps in to exaraded biquads i
prowated,

Figure 6 shows e block diagram of two cascaded biquads. The inpat
signal is labeled and the output sigual is labeled y, Internal signal
values are Inbeed ta andl with ef = 1, 24 — 2) etm — 2), nd
2.0 ~ 20 being delay signals obeained ftom preseling fer ope
tions, The general sequence of eperations used ly fer and obesin
requires nine additions snd eleven multiplications for uach value of
inpat ta the ter

Fire, y(n) ieobained by forming the sun

re Wo8 29 + by own = 1D AG extn

in the accumulator, rounding che eat, and aor i in & temporary
register. "The round resul soul). Thw prodacis are performed
the product registor before entering the werurnalator.

obtained in an hlentical meaner from the sum

tg #10 = BS ey ENF) + as 90)
hag Hn 2) 4 bus in,
Finally, fm} obtained from the rounded vale of the sums

in = 1 ais et

\VOICE-FAEQUENCY TRANSMISSION 1899
11] omen re ronconectcet

EEEE Beer

gtx the en od des a

[Before the next valuw of x enters the filter, eo(a ~ 2) is updated by
setting it waval roan ~ 1s wire ~ 5) ia updated with ae): tale — 2)
swith evn Tis and ete — 1) with ri)

“The outpaLof the receive path nqualizer is transmitted tn the digital
to-enalog converter and tothe cancel input.

622 concolor

Figure 5 shows a block digramn ofa 52¢ap transversal Mer. The
Desie operation in transversal fuer che multiplication of a tap
ty a delay ov‘put, followed by an addition in an sccura
i operation ean be accomplished with « sequence of inscuc

‘aq THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1861